There's no way a remap in itself will increase emissions for MOT purposes. If it's a petrol car then it's tested at idle so the fuelling and spark timing will be stock ordinary factory standard. A resistor pack plug in unit might cause a problem if it causes the engine to over fuel when it's warm.
Diesel: Same applies - diesels don't have a throttle valve as such so if a cheapie plug in unit is used the engine will idle at a higher RPM if it's led to over-fuel. Not decent MOT testing professional will test any car when the idle speed is higher than it should be.
I think that your friend has had his car re-mapped by a rock ape of an engineer and it will fail all emissions tests, eat fuel and drive badly.
A sympathetic remap is the finest single way to increase the performance of your car.
Done properly, the emissions will be the same for MOT purposes. OK, driven the way you want to drive (and the way I want to drive) you'll be slightly dirtier than a stock car. But, you will find, as I find, that a car remapped to deliver the performance envelope you want is more economical on fuel than a car in factory tune. It's all about driving style and journey types.
There's not point in having a performance driver in a car set up for maximum fuel economy at all costs. The driver will work it harder and thus burn more fuel. Much better to put said driver behind the wheel of a car that is set up to go properly.
My car is re-mapped (factory 136bhp) to 192 bhp and it walks through the emissions tests with equanimity. | 
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